Submesoscale Coastal Ocean Flows Detected by Very High Frequency Radar and Autonomous Underwater Vehicles

Abstract

Over a 29-day time series in July 1999, an ocean surface current radar (OSCR) in very high frequency (VHF) mode mapped the surface velocity field at 250-m resolution at 700 cells off Fort Lauderdale, Florida. During the experiment, autonomous underwater vehicles (AUVs), equipped with upward- and downward-looking 1.2MHz acoustic Doppler current profilers (ADCPs), measured subsurface current structure over four to six radar cells during two mixed layer patterns on 9 and 27 July 1999. As these AUV sampling patterns were conducted over 500 m 3 500 m and 500 m 3 750 m areas, these missions required about 80‐90 min (four radar sample intervals) to form four and seven synoptic snapshots, respectively. Based on autocorrelation analyses of the profiler data, along-AUV-track subsurface profiles were averaged at 10-s intervals, mapped to a surface from 1.5‐6.5 m, and compared to surface currents at more than 500 points for each snapshot. Comparisons between the surface and subsurface currents from the AUV revealed spatially averaged differences ranging from 4 to 26 cm s21 during these two experiments. The largest differences occurred when the surface and subsurface current vectors were orthogonal; otherwise, differences were O(10 cm s21). Scatterplots between 2-m and radar-derived surface currents indicated a consistent relationship with mooring data. From the seven spatial snapshots acquired during the second experiment, current profiles suggested a timedependent oscillation that was corroborated by radar and moored ADCP data. Least squares fits of these profiles from sequential AUV snapshots to a simple model isolated an ;9.2 6 1 h oscillation where the along-shelf current was O(50 cm s21). Spatially averaged current profiles from four and seven snapshots were subsequently time averaged to form a mean profile from each experiment. In the downwind directions, these mean profiles were compared to a winddriven, logarithmic layer profile in the upper 6.5 m based on a 10-m surface winds. Regression analyses suggest a slope of 1.16 between the theoretical and observed mean profiles with a bias of about 3 cm s21. In this context, the averaged winds played a role in driving the coastal ocean circulation. These results further suggest that the spatial averaging by the radar is consistent when subsurface current variations are averaged over similar time and space scales.